1. Field of the Invention
The present invention relates to optical fiber connectors, in particular optical fiber cable retention structures in optical fiber connectors.
2. Description of Related Art
There are many advantages of transmitting light signal via optical fiber waveguides, and the use thereof is diverse. Single or multiple fiber waveguides may be used simply for transmitting visible light to a remote location. Complex telephony and data communication systems may transmit multiple specific optical signals. These devices couple fibers in an end-to-end relationship, with the coupling being one source of light loss. Precision alignment of two polished ends of fibers is needed to ensure that overall optical loss in a fiber link is equal or less than the specified optical connector loss budget for a system. For single-mode telecommunication-grade fiber, this typically corresponds to connector fiber alignment tolerances that are less than 1000 nm. This means that in both parallel fiber and single fiber links, operating at multi-gigabit rates, the components applied to align the fibers must be assembled and fabricated with sub-micron precision.
In an optical fiber connection, an optical fiber connector terminates the end of a cable that contains one or multiple fibers, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so that light can pass end-to-end. Better connectors lose very little light due to reflection or misalignment of the fibers. Connectors, in both parallel/multiple fiber and single fiber links, operating at multi-gigabit rates must be assembled with subcomponents fabricated with sub micron precision. As if producing parts with such precision levels were not challenging enough, for the resulting end product to be economical, it must be done in a fully automated, very high-speed process.
Current optical fiber connectors have not changed in basic design for many years. The basic connector unit is a connector assembly. FIG. 1 illustrates an example of an optical fiber connector 1400 for a cable 1410 containing optical fibers 1412, which is commercialized by US Conec Ltd. The connector includes an assembly of components consisting of a ferrule 1402, a ferrule housing 1404, a cable boot 1406, alignment guide pins 1408, and other hardware provided within or outside the housing (e.g., cable strain relief, crimp, biasing spring, spacer, etc.). The ferrule 1402 and the terminating end faces of the fibers 1412 are polished. The ferrule 1402 in the optical fiber connector 1400 is spring-loaded to provide an axial bias to press together the polished end faces of the fibers in two connectors in an end-to-end configuration. In most cases, the intent is to establish physical contact between coupled fibers to prevent loss of light. Physical contact avoids a trapped layer of air between two fibers, which increases connector insertion loss and reflection loss. An adaptor, not shown, is required to securely couple the ferrules of two connectors (the ferrule housing 1404 of each connector is plugged into the adaptor).
The optical fiber connector illustrated in FIG. 1 manufactured by US Conec Ltd. is purportedly in accordance with the structure disclosed in U.S. Pat. No. 5,214,730, which is assigned to Nippon Telegraph and Telephone Corporation. As illustrated in the '730 patent, the optical fiber connector receives a optical fiber ribbon cable having a plurality of individual optical fibers and maintains the individual optical fibers in a predetermined relationship. The optical fiber connector can be mated with another optical fiber connector (e.g., using an adaptor) so as to align the plurality of individual optical fibers of one optical fiber connector with the plurality of optical fibers of the other optical fiber connector.
The ferrule 1402 from US Conec Ltd. is generally in the form of a plastic block having a series of over-sized through-holes that provide sufficient clearance for inserting the terminating ends of optical fibers 1412 and alignment pins 1408 into the block. The ferrule 1402 is formed by molding of a plastic polymer that is often reinforced by glass particles. To insert the terminating ends of the multiple optical fibers 1412 through the holes in the ferrule block 1402, the protective buffer jacket (resin) layers of the optic fibers are stripped off to expose the cladding layer near the terminating ends, and the cladding layer is coated with a layer of epoxy. The terminating ends of the optical fibers are then threaded into the over-sized holes in the ferrule. The ends of the optical fibers 1412 are securely held in the ferrule 1402 upon curing of the epoxy. Similarly, the alignment pins 1408 are retained with epoxy after inserting into the oversized holes in the ferrule 1402 provided for the pins.
The above described ferrule has several significant drawbacks. The injection molded structure inherently does not hold tolerance well. The polymer is not rigid and deforms when loads (forces or moments) are applied to the fiber cable or connector housing. Polymers are also susceptible to creep and thermal expansion/contraction over longer periods of time. The clearance in the over-sized holes in the ferrule further affects tolerance of end-to-end alignment of fibers. The epoxy shrinks upon curing, which leads to bending of the plastic ferrule. Further, epoxy creeps over time, leading to pistoning or retracting of the optical fiber ends (which are pushed against the ends of adjoining fibers) within the holes in the ferrule under the applied axial bias of the spring-load in the connector. This compromises the integrity of the surface contact interface of opposing fiber end faces. These and other deficiencies result in poor resultant tolerance that is more to be desired for modern day optical fiber applications.
Currently, it is generally accepted that fiber connectors cost too much to manufacture and the reliability and loss characteristics are more to be desired. The tolerance of the fiber connectors must improve, and the cost of producing fiber connectors must decrease if fiber optics is to be the communication media of choice for short haul and very short reach applications. The relatively widespread and ever increasing utilization of optical fibers in communication systems, data processing and other signal transmission systems have created a demand for satisfactory and efficient means of inter joining fiber terminals.
To address those needs, the Assignee of the present invention developed a series of optical fiber connector designs, which overcome many of the drawbacks of the prior art. Some of these designs are directed to various improvements to ferrule designs (e.g., ferrules with open grooves and ferrules having alignment structures with open grooves), which can be more cost effectively manufactured and result in optical fiber connectors that are more reliable compared to the prior art optical fiber connectors noted above.
To further push the envelope in optical fiber connector designs, it is further desirable to improve on other aspects of optical fiber connectors beyond ferrules designs. One aspect that can be improved is the retention structure for the fiber cable to prevent breakage of the fiber and retraction of the optical fiber from the ferrule. In U.S. Pat. No. 7,311,449, FIGS. 38 and 39 refers to a crimping member attached to a fiber ferrule. A fiber strength member extends from the crimping member, which supports the fiber cable and maintains the longitudinal position of the fiber relative to the ferrule. The fiber strength member provides strain relief for the fiber cable. It is noted that given the soft materials of the buffer jacket of an optical fiber cable, the overall outer diameter (O.D.) of the fiber cable does not have a tight dimensional tolerance. For example, according to a particular specification for tight buffer optical fiber cables published by Corning Cable Systems in April 2012, the external diameter of the thermoplastic buffer coating shall be 900±50 μm. Heretofore, retention structures for the fiber cable have not been designed to effectively accommodate the relatively large ±50 μm variation of the buffer jacket.
What is needed is an improved optical fiber connector having an improved fiber cable retention structure, which accommodates large variations in overall fiber cable external diameter, and further improves manufacturing, ease of use and reliability at reduced costs.